In general, microwave heating ovens are frequently utilized for heating articles including foods within a comparatively short time. For heating articles in an oven microwaves of 2450 MHz are usually generated. The high frequency waves thus generated, i.e. microwaves, are apt to leak out from the gaps between the oven body and the door. Such leakage of microwaves causes disturbance of electromagnetic waves in the household appliances, such as radio and TV and may cause harm to the human body. For such reasons, the use of the microwave absorber, which can prevent the leakage of microwaves from the gap between the oven body and the door of microwave heating ovens is needed.
Until now, many patents disclosed means for preventing the leakage of microwaves from microwave heating ovens. For example, U.S. Pat. Nos. 3,742,176 and 3,866,009 disclosed the use of ferromagnetic materials prepared by mixing ferrite powder of formula MFe.sub.2 O.sub.4, wherein M is Ni, Cu, Zn, Mn or Mg, with a high molecular weight organic polymers selected from natural rubbers and synthetic rubbers.
In U.S. Pat. No. 4,046,983 there was reported a method for preparing microwave absorbers which comprises mixing a ferrite powder with an organic high molecular weight materials selected from synthetic rubbers, thermoplastic resins and thermosetting resins; and in U.S. Pat. No. 4,003,840 a method for preparation of microwave absorbers by mixing a ferrite powder having a particle size of less than 1.65 mm and the general formula MFe.sub.2 O.sub.4 in which M is Mn, Ni, Cu, Zn, Mg or Co, with an organic high molecular weight material selected from the group consisting of thermosetting resins (phenolic resins, polyester resins, epoxy resins, silicone resins), thermoplastic resins (PVC, polyethylene, polypropylene), and natural and synthetic rubbers (polychloroprene, acrylonitrile-butadiene-styrene, fluorine-containing rubbers), has been disclosed. In addition, U.S. Pat. No. 4,602,141 proposed a method for producing a microwave absorber which comprises mixing ferrite powder (Mn,Zn,Fe-ferrite powder or Mn,Cu,Zn-ferrite powder), carbon powder and a high molecular polymer.
However, the microwave absorbers produced by utilizing these conventional methods and compositions do not provide a satisfactory level of heat-resistance and mechanical strength. Particularly, when the microwave absorber is mounted on the microwave heating oven and used for a long time, the durability of the microwave absorber is significantly reduced according to the accumulation of operating time thereof, i.e. with the passage of time in which the microwave heating oven is operated. Thus, there is a drawback in that the microwave absorbing effect (attenuation effect) or heat energy insulating effect is relatively lowered.
Accordingly, the present inventors have extensively researched microwave absorbers providing the solution to the aforementioned problems and thus now have invented a microwave absorber having improved microwave absorbing effect (attenuation effect) and heat energy insulating effect from those of the conventional absorbers as well as more improved heat-resistance and mechanical strength.